324 CHARLES PACKARD 



matter. The latter are all absorbed by 2 mm. of lead, or by 250 

 cm. of air. Thus it is seen that the distance of the object to 

 be exposed from the radium is a large factor in determining the 

 intensity of the radiation. The slower beta particles are easily 

 absorbed by thin layers of matter much less dense than lead. A 

 thick mica screen is sufficient to stop them, although they pass 

 through a very thin screen of mica without much loss of energy. 

 Chemically, the beta rays are much less active than the alpha 

 rays, since they are not absorbed as readily. It is therefore ap- 

 parent that the slower beta rays are more active than the more 

 rapid ones since they are absorbed in passing through matter. 

 This point is of importance in conducting an experiment for if 

 these more active rays are unable to reach the object of experiment 

 the effects produced may be very different from those obtained 

 when all of the beta rays are available. 



The gamma rays are similar to the hard X-rays in many 

 respects, but they travel with much greater velocity. For this 

 reason they are not absorbed to any extent and their effects 

 are of a different order from those produced by the beta rays. 

 In a magnetic field they are not deflected. When they pass 

 through lead of considerable thickness they are transformed into 

 secondary beta rays, similar to the beta rays emitted by radium 

 itself. These rays have been shown by Congdon to produce 

 definite effects on living matter. 



REVIEW OF LITERATURE 



Up to the present time very few studies have been made on 

 the effectiveness of the different kinds of rays, or on the relative 

 effects of the slow and rapid beta rays. Guilleminot ('07) 

 showed that the beta rays are more effective than X-rays when 

 their luminescent effects are approximately the same. A more 

 thorough study is that of Congdon ('12) in which he analyzed 

 the effects of the primary and secondary beta rays on seedlings. 

 He states that the gamma rays from 8 mg. of radium bromide 

 produce no appreciable effect. Of the beta rays, the slow 

 electrons are more effective in retarding growth that are the 

 rapid electrons. 



